With a gruesome crash to earth, an unimaginable scenario became all too real.

A university student was sunbaking in his south London backyard last week when out of the literal clear blue sky, a frozen body landed just centimetres from him with such almighty force it left a crater in the garden.

The body was that of a mysterious stowaway, who presumably snuck into the landing gear of a Kenyan Airways plane and fell as the plane prepared to land at Heathrow Airport.

Death is almost guaranteed for stowaways — people who sneak onto flights undetected, typically by hiding in wheel compartments in the undercarriage of planes.

If the lack of oxygen during the flight doesn't kill them, the freezing conditions at high altitude will. Or being crushed by the retracting wheels of the plane or falling out when the compartment reopens to release the wheels for landing. That's if the stowaway even gets away with the massive security breach in the first place.

Some stowaways have reached their destination alive, but the odds favour death. And that's exactly the fate of a Sydney schoolboy 50 years ago.

STORY BEHIND THIS HORRIFIC PHOTO

Young Keith Sapsford, from Randwick in Sydney's east, was "a wanderer". Restless. Always on the move.

His parents had taken him on an overseas holiday to satisfy his lust for adventure, but it only made his travel bug worse, the Sydney Morning Herald reported in 1970.

When he was 14, his parents sent him to Boys' Town, a Catholic home for teenagers in the southern Sydney suburb of Engadine.

After a couple of weeks at Boys' Town, Keith ran away.

Just a few months earlier, Keith's dad CM Sapsford, a university lecturer in mechanical and industrial engineering, told him the tragic story of a boy in Spain who died hiding in the undercarriage of an aircraft. Sapsford explained the dangers of exposure to high altitude and the moving parts of the plane. But Keith's mind was set.

On February 22, 1970, three days after running away from Boys' Town, Keith snuck onto the tarmac at Sydney Airport. He climbed up in the wheel compartment of a Douglas DC-8 bound for Tokyo and waited until the plane took off.

At the same time, unaware of the tragedy that was about to unfold before him, amateur photographer John Gilpin was taking photos at the airport. He accidentally captured the precise moment Keith fell about 46 metres from the plane as it took off.

In fact, Gilpin wasn't even aware of the tragedy while it was happening. It wasn't until a week later, when he was developing the photos, he saw the figure of a boy falling from the plane, feet-first, with his hands up near his head.

Keith died from falling when the door to the plane's wheel compartment opened. Police determined he didn't realise the compartment would open when the airborne plane's wheels retracted.

'FOOLISH, IGNORANT AND COMPLETELY DESPERATE'

Retired Boeing 777 captain Les Abend saw a lot in his 34-year career as a pilot, but he said there was one thing that never ceased to amaze him: "That people will actually stow away inside the landing gear well of a commercial airliner and expect to survive."

Writing for CNN this week, he said: "Any individual who attempts such a feat is foolish, ignorant of the dangerous situation — and must be completely desperate."

One in four plane stowaways survive the perilous journey, according to 2015 figures from the US Federal Aviation Authority, including a 20-year-old who hid in the undercarriage of a private jet and made it from Vienna to London in 2010. Successful cases typically involve very short journeys when the plane is flying lower than usual cruising altitude.

In 2015, one of two men who stowed themselves on a British Airways flight from Johannesburg to London managed to complete the journey with his life but ended up in hospital in a serious condition. The other man died.

In 2000, a man made it from Tahiti to Los Angeles alive and, in 2002, so did another man from Cuba to Canada — but both arrived at their destinations with severe hypothermia.

Between 1947 and 2012, there were 96 known stowaway attempts in wheel compartments of 85 flights. Of those stowaways, 73 died and only 23 survived.

HOW, AND WHY, DOES THIS HAPPEN?

The number, albeit small, of successful stowaways is even more astonishing given how robust airport security makes it difficult for someone to sneak onto the plane in the first place. Most successful attempts originate at airports with poor security — but not all.

"Pilots use flashlights on their walk-around inspections, but they can't see every nook and cranny," Abend said.

"Often the folks loading the bags or servicing the aeroplane are focused on their task and may not catch an unauthorised person on the ramp. If that individual had remained hidden from view, he may have waited until the aeroplane was not being loaded or serviced."

Kenya's civil aviation authorities said the stowaway who fell from the plane over London this week most likely had legal access to Nairobi's Jomo Kenyatta International Airport, as tight security meant an outsider was unlikely to have crossed the runway unnoticed.

"They do check every part of the aeroplane, including the undercarriage, the wheels, the brakes, the tyre condition, the wheel well that is above there," Kenya Civil Aviation Authority director general Gilbert Kibe said.

"They inspect everything. So when those checks were being done, it is not likely that person was there, otherwise he would have been seen.

"So at which point the person gained access, that is the mystery."

The London Heathrow flight path is no stranger to stowaway tragedies. In another case, in 2012, a man trying to reach the UK from Angola fell from a plane and landed on a footpath in southwest London. A coroner found he had died due to hypothermia and lack of oxygen.

Almost all stowaways are male. Some cases have been attributed to a misguided sense of adventure, as in the case of Sydney's Keith Sapsford, but most appear to be acts of desperation.

"We don't know the circumstances of these particular people, but we know from our work with refugees that people are often forced to take extreme measures in order to flee their countries," former Refugee Council chief operating officer Deborah Harris told the BBC in 2012.

"In conflict situations, people often have to leave their homes at very short notice and may have no access to money or belongings so are forced to take desperate measures to escape."

Aviation expert David Learmount told the BBC the chances of survival were so low it was never worth the risk.

"They either get crushed or frozen to death," he said. "There's a huge degree of ignorance. If anyone knew what they were letting themselves in for they wouldn't do it."